build: make GPG signing conditional in PublishingConventionPlugin for local publishing

[dkhawk]

What this PR does

When developers run ./gradlew publishToMavenLocal on their local machines without configuring GPG signing credentials (signing.keyId, signing.secretKeyRingFile), the build fails with:

Invalid publication 'maven': artifact file does not exist: '.../release-javadoc.jar.asc'

This occurred because signAllPublications() in PublishingConventionPlugin.kt was invoked unconditionally.

This PR wraps signAllPublications() so that it only signs when signing.keyId or signing.secretKeyRingFile is present and non-empty:

val isSigningConfigured = !providers.gradleProperty("signing.keyId").getOrElse("").isEmpty() ||
    !providers.gradleProperty("signing.secretKeyRingFile").getOrElse("").isEmpty()
if (isSigningConfigured) {
    signAllPublications()
}

Verification

• Executed ./gradlew publishToMavenLocal across all modules: BUILD SUCCESSFUL without .asc signature errors.
• Verified that all 7 library modules are correctly installed in ~/.m2/repository/com/google/maps/android/.

[googlemaps-bot]

Code Coverage

Overall Project	42.99%	🍏

There is no coverage information present for the Files changed

[kikoso]

Thanks for this @dkhawk, and sorry it sat open so long. The underlying problem was real, but it has since been fixed on main independently, so this PR is no longer needed.

ca970251f (#1766, the Kotlin migration PR, landed 2026-08-28) added an equivalent conditional at the same spot in PublishingConventionPlugin.kt:

publishToMavenCentral()
if (findProperty("signing.keyId")?.toString()?.isNotBlank() == true ||
    findProperty("signing.secretKeyRingFile")?.toString()?.isNotBlank() == true ||
    findProperty("signingInMemoryKey")?.toString()?.isNotBlank() == true
) {
    signAllPublications()
}

So ./gradlew publishToMavenLocal without GPG credentials already succeeds on main today. That overlap is also why this PR now shows as conflicting.

Two reasons I'd keep the version on main rather than rebasing this one onto it:

1. It also checks signingInMemoryKey, vanniktech's in-memory signing path. Our publish.yml currently uses the keyring-file route (it seds signing.keyId and signing.secretKeyRingFile into gradle.properties), so this PR would not break the current release, but it would drop support for the in-memory path that main deliberately covers.
2. isNotBlank is a little safer than !...isEmpty() here, since gradle.properties ships these keys with empty values on lines 29-31 and CI rewrites them via sed. A stray space would flip an isEmpty check to true and reintroduce the original .asc failure.

One idea from here worth keeping though: providers.gradleProperty(...) is the configuration-cache-friendly Provider API, whereas findProperty reads project state at configuration time, which is exactly the pattern that trips up the configuration cache inside a convention plugin. If you want to land that separately, it is a small follow-up on main:

val signingProps = listOf("signing.keyId", "signing.secretKeyRingFile", "signingInMemoryKey")
if (signingProps.any { providers.gradleProperty(it).getOrElse("").isNotBlank() }) {
    signAllPublications()
}

Closing as already fixed.